In general, an organic light emitting diode display is a display device that electrically excites fluorescent organic material for emitting light and visualizes an image by voltage programming or current programming of N×M numbers of organic light emitting cells.
Such organic light emitting cells are called organic light emitting diodes (OLED) because they have a diode characteristic. An OLED includes anode (e.g., indium tin oxide), organic thin film, and cathode (metal) layers. The organic thin film has a multi-layer structure, including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL), to balance electrons and holes and thereby enhance light emitting efficiency. Further, the organic thin film separately includes an electron injection layer (EIL) and a hole injection layer (HIL). The N×M organic light emitting cells arranged in the matrix format form an OLED display panel.
Methods of driving the organic light emitting cells having the foregoing configuration include a passive matrix method and an active matrix method employing a thin film transistor (TFT) or a MOSFET. In the passive matrix method, an anode and a cathode are formed crossing each other and a line is selected to drive the organic light emitting cells. However, in the active matrix method, an indium tin oxide (ITO) pixel electrode is coupled to a TFT and the light emitting cell is driven in accordance with a voltage maintained by capacitance of a capacitor. Hereinafter, a pixel circuit of a general active matrix OLED display will be described. FIG. 1 is a diagram of an equivalent pixel circuit of the first pixel of the N×M pixels (i.e., a pixel positioned at the first column and the first row).
As shown in FIG. 1, a pixel 10 includes subpixels 10r, 10g, and 10b. The subpixels 10r, 10g, and 10b respectively include organic light emitting diodes OLEDr, OLEDg, and OLEDb emitting red (R), green (G) and blue (B) lights. In a striped structure of subpixels, the subpixels 10r, 10g, and 10b are coupled to different data lines D1r, D1g, and D1b, respectively and are coupled to a common scan line S1.
The subpixel 10r of red light includes two transistors M1r and M2r, and a capacitor C1r for driving the OLEDr. Similarly, the subpixel 10g of green light includes two transistors M1g and M2g, and a capacitor C1g, and the subpixel 10b of blue light includes two transistors M1b and M2b, and a capacitor C1b. The operations of these subpixels 10r, 10g, and 10b are the same, therefore, only the operation of the subpixel 10r will be described.
The driving transistor M1r is placed between a source voltage VDD and an anode of the OLEDr, and coupled thereto. The transistor M1r and transmits a current for light emission to the OLEDr. A cathode of the OLEDr is coupled to a voltage VSS that is lower than the source voltage VDD. A current passing through the driving transistor M1r is controlled by a data voltage applied by the switching transistor M2r. Here, the capacitor C1r is coupled to a source and a gate of the transistor M1r and sustains an applied voltage. The gate of the transistor M2r is coupled to the scan line S1 through which a scan signal containing on/off information is transmitted. The source of the transistor M2r is coupled to the data line D1r through which a data voltage for the red subpixel 10r is transmitted.
As described in the above, in the OLED display, one pixel 10 includes three subpixels 10r, 10g, and 10b, and each subpixel includes a driving transistor, a switching transistor, and a capacitor for driving an OLED. On each subpixel, a data line for transmitting data signals and a source line for transmitting the source voltage VDD are formed. Accordingly, in the pixel, a large number of transistors, capacitors, and wires for transmitting voltages and signals are necessary, and thus it is difficult to arrange all of the components in the pixel. Moreover, in order to decrease the aperture ratio of an OLED, the light emission area needs to become smaller.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and therefore, unless explicitly described to the contrary, it should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.